The present invention relates to a solid electrolytic capacitor containing an electrically conducting carbon paste favored with good filling property and high electrical conductivity, having heat resistance, small in thermal deterioration, and high in the humidity resistance. More specifically, the present invention relates to a solid capacitor obtained by forming a solid electrolyte layer on a dielectric film on the surface of a valve-acting metal having fine pores and forming further thereon an electrically conducting carbon paste layer and an electrically conducting metal powder paste layer. The binder of the electrically conducting carbon paste is allowed to infiltrate into the inside of the solid electrolyte layer or into both the inside of the solid electrolyte layer and the inside of fine pores, so that the solid electrolytic capacitor can be improved in the adhesion between the electrically conducting material with the dielectric film and the solid electrolyte layer and can have mechanical strength, high capacitance, low impedance, good humidity resistance load property and excellent heat resistance. The present invention also relates to a method for producing the solid electrolytic capacitor.
A solid electrolytic capacitor device is generally manufactured by forming an oxide dielectric film layer on an anode substrate comprising a metal foil subjected to an etching treatment to have a large specific area, forming on the outer side thereof a solid semiconductor layer (hereinafter simply referred to as a xe2x80x9csolid electrolytexe2x80x9d) as a counter electrode, and preferably further forming an electrically conducting layer such as electrically conducting paste. After completely sealing the whole device with epoxy resin or the like, the device is used as a capacitor part for electric products in a wide range of fields.
In recent years, to satisfy the requirements for digitization of electric instruments or higher processing speed of personal computers, there has been a demand for the solid electrolytic capacitor to have a small size, a large capacity and low impedance characteristics in the high frequency region.
To cope with these requirements for the solid electrolytic capacitor, an electrically conducting polymer is applied as the solid electrolyte, thereby bringing out a great effect on the improvement of capabilities of the capacitor device. However, this is still insufficient and there is a demand for the electrically conducting paste covering the outer surface of the solid electrolyte of a capacitor device to have improved capabilities.
The electrically conducting material in the electrically conducting paste used in a solid electrolytic capacitor is a metal powder such as gold, silver or copper. Among these, silver powder is widely used in view of capability. However, silver causes migration and therefore, when it is used, for example, for a solid electrolytic capacitor, it is necessary to previously coat an electrically conducting carbon paste and coat thereon the electrically conducting silver paste.
A large number of proposals have been made with respect to the electrically conducting material, binder and solvent constituting the electrically conducting carbon paste.
With respect to the electrically conducting material for the electrically conducting carbon paste, for example, use of a combination of natural graphite (10 to 20 xcexcm) and carbon black (see, JP-A-9-31402 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)), use of carbon having a particle size of tens of microns with an attempt to bring out an anchoring effect by the particles projecting from the electrically conducting carbon layer (see, JP-A-5-7078), use of carbon black of 20 xcexcm or less and a synthetic resin as a combination of electrically conducting material and binder (see, JP-A-4-181607), use of graphite powder flake, graphite fine powder (aspect ratio: 10 or more, average particle size: 10 xcexcm or less) and epoxy resin (see, JP-A-7-262822), use of graphite and a fluorine-containing polymer such as PTFE fine particle (see, JP-A-61-69853), and use of carbon powder and a glycidyl ether as a combination of electrically conducting material and solvent (see, JP-A-4-177802) have been proposed.
For the synthetic resin as the binder, polyethylene, epoxy resin and phenol resin have been proposed.
In electrically conducting carbon pastes used in over a wide range of fields, the above-described natural graphite or carbon black is used and epoxy resin is used as the binder.
However, the electrically conducting carbon paste using natural graphite has problems in that the filling property (adhesive property) is bad because the natural graphite is in the scale form. In addition, the electrical conductivity is low due to the presence of many impurities, and since the surface after the coating has little asperities, layer separation readily occurs at the interface or the impedance easily deteriorates by heat.
The electrically conducting carbon paste using carbon black has similar problems as in the case using natural graphite. That is, the filling property cannot be increased because the powder particle is very small and therefore, the electric conductivity cannot be elevated. Furthermore, another problem with the electrically conducting carbon paste using natural graphite or carbon black is that a dispersion treatment is necessary in the manufacture of the paste.
On the other hand, the epoxy resin is advantageous when used as the binder because the cost is low and the handling is easy; however, there still is a problem, such as high rigidity, low capability of relaxing the reduction in the stress generated between the chip and the lead frame at the heating in the reflow soldering treatment, which is attributable to the formation of a large-size chip, and easy occurrence of deterioration in the moisture resistance due to high water absorptivity.
With respect to the improvement of the adhesive property between the solid electrolyte and the electrically conducting paste layer, a solid electrolytic capacitor comprising a valve-acting metal anode having formed thereon an oxide film layer, a semiconductor layer, an electrically conducting carbon layer comprising a non-aqueous resin, a heat-sensitive inorganic powder and an electrically conducting carbon powder and a cathode electrically conducting layer in this order has been proposed (see, JP-A-62-8513). In this case, strong adhesion is achieved between the non-aqueous resin containing electrically conducting carbon powder and the manganese dioxide layer as the solid electrolyte. Therefore, the impedance can be prevented from increasing in the tests of moisture resistance and heat resistance.
Furthermore, JP-A-2-260525 describes a method for producing a solid electrolytic capacitor, where chemically formed foils (anode foil and cathode foil) each having a surface oxide film formed by electrolytic oxidation are used. A polypyrrole polymerization film is formed by chemical polymerization and electrolytic polymerization on the anode foil and the cathode foil, these foils are coiled through a porous separator, an electrically conducting paste is impregnated into the porous separator to manufacture a device, and the device is sealed to obtain a product. In this case, the polypyrrole polymerization film is contacted with the porous separator impregnated with the electrically conducting paste over a wide area, so that the strength is improved and so that a problem in the reliability due to uncertainty in the adhered area, which is encountered in the case of using an electrically conducting paste and taking out the cathode, can be overcome.
According to the technique described in JP-A-62-8513, by virtue of the strong adhesion between the non-aqueous resin containing electrically conducting carbon powder and the manganese dioxide layer as the solid electrolyte, the impedance can be prevented from increasing in the tests of moisture resistance and heat resistance. However, the solid electrolyte is limited to manganese dioxide and in the case of a solid electrolytic capacitor using an electrically conducting polymer as the solid electrolyte, sufficiently strong adhesion is not guaranteed.
According to the technique described in JP-A-2-260525, it is necessary to use a porous separator and at the same time allow the polypyrrole polymerization film to contact the porous separator impregnated with an electrically conducting paste over a wide area, and in this technique, the adhesive property is not referred to except for the contact area.
The present inventors have found that in a solid electrolytic capacitor obtained by forming an electrically conducting polymer as a solid electrolyte layer on a dielectric film on the surface of a valve-acting metal having fine pores and forming thereon an electrically conducting carbon paste layer and an electrically conducting metal powder paste layer, the adhesion to the electrically conducting metal material layer and the adhesion between the dielectric film and the solid electrolyte are improved and a high-performance solid electrolytic capacitor can be obtained when the binder of the electrically conducting carbon paste is allowed to infiltrate into the inside of fine pores and into the solid electrolyte layer formed thereon and particularly when a material having rubber elasticity is used as the binder component. The present invention has been accomplished based on this finding.
More specifically, the present invention provides a solid electrolytic capacitor and a production method therefor described below.
(1) A solid electrolytic capacitor comprising a dielectric film on the surface of a valve-acting metal having fine pores, a solid electrolyte layer, an electrically conducting carbon paste layer and an electrically conducting metal powder paste layer in order, wherein the binder of the electrically conducting carbon paste is allowed to infiltrate into the solid electrolyte layer.
(2) A solid electrolytic capacitor comprising a dielectric film on the surface of a valve-acting metal having fine pores, a solid electrolyte layer, an electrically conducting carbon paste layer and an electrically conducting metal powder paste layer in order, wherein the binder of the electrically conducting carbon paste is allowed to infiltrate into the solid electrolyte layer and into the inside of the fine pores of the valve-acting metal.
(3) The solid electrolytic capacitor as described in (1) or (2) above, wherein the binder of the electrically conducting carbon paste comprises a material which is softened at a temperature of 330xc2x0 C. or less, can swell or suspend in the solvent of the paste and has rubber elasticity.
(4) The solid electrolytic capacitor as described in (3) above, wherein the material having rubber elasticity is at least one material selected from the group consisting of isoprene rubber, butadiene rubber, styrene/butadiene rubber, nitrile rubber, butyl rubber, ethylene/propylene copolymer, acrylic rubber, polysulfide rubber, fluorine-containing polymer, silicone rubber and thermoplastic elastomer.
(5) The solid electrolytic capacitor as described in any one of (1) to (4) above, wherein the electrically conducting carbon paste comprises solid contents consisting of from 30 to 99% by mass of the electrically conducting material and from 1 to 70% by mass of the binder.
(6) The solid electrolytic capacitor as described in (5) above, wherein the electrically conducting material is a material containing 80% by mass or more of artificial graphite.
(7) The solid electrolytic capacitor as described in (6) above, wherein the artificial graphite has a fixed carbon content of 97% by mass or more, an average particle size of 1 to 13 xcexcm and an aspect ratio of 10 or less, and contains 12% by mass or less of particles having a particle size of 32 xcexcm or more.
(8) The solid electrolytic capacitor as described in any one of (1) to (7) above, wherein at least a part of the solid electrolyte layer has a lamellar structure.
(9) The solid electrolytic capacitor as described in (8) above, wherein the solid electrolyte layer has a space portion at least in a position between layers of the lamellar structure.
(10) The solid electrolytic capacitor as described in (8) or (9) above, wherein the solid electrolyte having a lamellar structure has a thickness of 0.1 to 0.3 xcexcm per layer.
(11) The solid electrolytic capacitor as described in any one of (1) to (10) above, wherein the valve-acting metal is selected from the group consisting of aluminum, tantalum, niobium, titanium, zirconium and alloys thereof.
(12) The solid electrolytic capacitor as described in any one of (1) to (11) above, wherein the solid electrolyte layer comprises an electrically conducting polymer and a monomer for forming the electrically conducting polymer is a compound containing a 5-membered heterocyclic ring.
(13) The solid electrolytic capacitor as described in any one of (1) to (11) above, wherein the solid electrolyte layer comprises an electrically conducting polymer and a monomer for forming the electrically conducting polymer is a compound having an aniline skeleton.
(14) The solid electrolytic capacitor as described in (12) above, wherein the compound containing a 5-membered heterocyclic ring is a compound selected from the group consisting of pyrrole, thiophene, furan, polycyclic sulfide and substitution derivatives thereof.
(15) The solid electrolytic capacitor as described in (14) above, wherein the compound containing a 5-membered heterocyclic ring is a compound represented by the following formula (I): 
(wherein the substituents R1 and R2 each independently represents a monovalent group selected from the group consisting of hydrogen, a linear or branched, saturated or unsaturated hydrocarbon group having from 1 to 10 carbon atoms, an alkoxy group, an alkyl ester group, a halogen, a nitro group, a cyano group, a primary, secondary or tertiary amino group, CF3, a phenyl group and a substituted phenyl group; the hydrocarbon chains of R1 and R2 may combine with each other at an arbitrary position to form a divalent chain for forming at least one 3-, 4-, 5-, 6- or 7-membered saturated or unsaturated hydrocarbon ring structure together with the carbon atoms to which those hydrocarbon groups are substituted, and the ring connecting chain may arbitrarily contain a bond of carbonyl, ether, ester, amide, sulfido, sulfinyl, sulfonyl or imino).
(16) The solid electrolytic capacitor as described in (15) above, wherein the compound containing a 5-membered heterocyclic ring is a compound selected from the group consisting of 3,4-ethylenedioxythiophene and 1,3-dihydroisothianaphthene.
(17) A method for producing a solid electrolytic carbon capacitor, comprising the steps of forming a dielectric film on the surface of a valve-acting metal having fine pores; forming a solid electrolyte layer on the dielectric film; forming an electrically conducting carbon paste layer comprising an electrically conducting carbon material, a binder capable of being softened at a temperature of 330xc2x0 C. or less and having a rubber elasticity, and a solvent and an electrically conducting metal powder paste layer.
(18) The method for producing a solid electrolytic capacitor described in (17) above, further comprising the step of allowing the binder of the electrically conducting carbon paste to infiltrate into the solid electrolyte layer.
(19) The method for producing a solid electrolytic capacitor described in (17) above, further comprising the step of allowing the binder of the electrically conducting carbon paste to infiltrate into the solid electrolyte layer and into the inside of fine pores of the valve-acting metal.